One detail among the many reports emerging about Aaron Alexis, the 34-year-old man suspected of killing 12 people in a shooting at the Washington Navy Yard on Monday, stood out: he was a regular meditator.

How does someone who engages in meditation, which is supposed to focus the mind, and is often associated with efforts to diffuse violence, rather than instigate it, perform the acts that Alexis is accused of executing? Alexis had a record of violent crime and, his father told the Wall Street Journal that his son had anger issues related to post-traumatic stress from participating in rescue efforts during the 9/11 attacks. A former boss, who met Alexis at a Buddhist temple in the Fort Worth, Tex. area, said Alexis was also a heavy drinker who came to chanting and meditation sessions regularly.

At worst, most people see meditation as flaky, boring, self-involved or harmless. But as research starts to document how it can help to fight stress, high blood pressure, addictions and many other mental and physical disorders, it’s also becoming clear that meditating isn’t always so benign — particularly if it’s used against a background of existing mental illness.

People with depression or past experiences of trauma, for example, may find themselves feeling increasingly anxious during meditation, no matter how much they try to focus on the moment. Or they may be plagued by intrusive thoughts, feelings and images of the past during their mindfulness exercises.

That’s why [University of Washington researcher Sarah] Bowen suggests that people with depression or trauma issues who want to benefit from meditation should try it with expert guidance. “If you get stuck in ruts like rumination, there are ways to work with that,” she says, “It’s important to have teachers who are very familiar with meditation to guide you as you are learning.” Experts can let people know what to expect and offer emotional support to help them through rough patches.

Brown University neuroscientist Dr. Willoughby Britton, who has published research demonstrating how meditation can be used in depression, is currently carrying out what she calls the “dark night” project, which will explore the rockier parts of the mindfulness path.

Britton was inspired to do the research in part by two patients she treated during her psychiatry residency. Both were participating in a meditation retreat had to be hospitalized for symptoms they developed during their contemplation. She later attended a retreat— and experienced for herself what it was like to follow meditation into an extreme and distressing mental state. As she described it in an online interview, “I thought that I had gone crazy. I thought I was having a nervous breakdown. I mean I really had no idea why I was suddenly having all these…like terror was big symptom of mine.”

She eventually learned that overwhelming anxiety, fear and emotional pain— sometimes including symptoms severe enough to merit psychiatric diagnosis— are “actually classic stages of meditation” that eastern practitioners are familiar with. But Western doctors and researchers who co-opted the practice and began advocating meditative techniques to treat mental illness were not studying them. They saw only the calming ability of meditation to focus the mind.

Although Britton’s research is not yet published, there are enough anecdotes about such dark experiences in writings on meditation and from teachers and practitioners to suggest caution in prescribing the practice for the severely mentally ill without appropriate guidance.

While it’s impossible to know what role, if any, meditation played in Alexis’ mental states, it’s clear that most therapies and practices that are powerful enough to have positive effects are also capable of doing harm when used in the wrong way and in the wrong people.